1. Field of the Invention
This invention relates generally to optical telecommunications and data transmission systems, and more particularly to optical demodulation including devices for optical heterodyning in a single semiconductor structure.
2. Background of the Invention
Coherent detection of modulated optical signals provides a significant theoretical advantage over direct detection with respect to the amount of optical power required to maintain a required error rate. In practice, it is difficult to achieve the potential benefits of coherent detection for several reasons. First, coherent detection requires the use of a spectrally pure local oscillator laser running at a frequency displaced from the signal frequency by an amount equal to the desired intermediate frequency. Secondly, the wavefront of the signal wave and the local oscillator wave must be matched to less than a small part of the optical wavelength over the beam falling on the sensative area of a photodetector. Finally, the alignments in position and frequency must be carefully maintained over time for proper operation. These practical problems are formidable, and have delayed the wide acceptance of coherent laser communications.
In the telecommunications field the benefits of coherent detection are sufficiently attractive such that efforts to design practical systems have gone forward. For example, by injecting the received signal and the local oscillator through a fiber coupler, the wave front matching condition can be automatically satisfied, and the problem of alignment stability overcome. However, a laser oscillator, fiber coupler and detector are still required, and no preamplification is obtained.